1. Field of the Invention
This invention relates to an electrooptical display screen as well as to a method of fabrication of the screen and is more particularly concerned with a matrix-control flat-panel display screen. The invention is applicable to the fabrication of large-area liquid crystal flat-panel screens in which the control points are formed by thin-film integration.
2. Description of the Prior Art
It is already known that display screens usually have a large number of image points or elements of square or rectangular shape. These image elements have to be addressed individually. The definition of the screen is a function of the number of points which are capable of receiving an item of information. Control of each point is performed by application of an electric field through the liquid crystal. For the purpose of visualizing alphanumeric or graphic data, it has been proposed to provide matrix-type displays. In this design, each image element is defined by the intersection of two orthogonal arrays of leads designated as rows and columns.
Addressing of matrix-type display screens assumes steadily greater importance as it is sought to attain higher levels of definition or in other words to increase the number of elementary image points.
Since the elementary points are addressed sequentially line by line, the number of addressable lines is usually limited by the characteristics of the electrooptical effect of the liquid crystal employed. The possibility of addressing a large number of lines (&gt;100) is accordingly carried into effect at the expense of the other characteristics of the screen (reduction in contrast and increase in angular dependence).
In order to improve the performances of these screens, it is possible to mount in series with each image element a nonlinear element whose electrical impedance varies:
either as a function of the voltage applied to its terminals by making use of a device such as a zinc oxide varistor as described in the article by A. C. Castleberry et al. published in SID 1980 Technical Digest or else by making use of a MIM element as described in the article by K. Niwa et al. published in SID 1984 Technical Digest or else by employing head-to-tail diodes as described in the article by N. Szydlo et al. published in Japan Display 83 Technical Digest;
or as a function of a control voltage by employing thin-film transistors as described in the article by T. P. Brody et al. published in IEEE Electron Devices ED 20 995 (1973).
In the field of display screens, current technical requirements are concerned with the achievement of optimum image definition. In the case of screens of the matrix display type, it is accordingly found necessary to design devices having a large number of addressing rows or columns which can amount to as many as 1024 or even more. This entails a corresponding increase in the number of control elements. For purposes of large-scale manufacture, it is necessary in particular to obtain high production efficiency, good reproducibility and high stability of these components. Moreover, the electrical characteristics of the component must necessarily be matched with those of the associated cell, also with good reproducibility.